The invention relates to control of excessive and abnormal displacements of the talocrural and subtalar joints of the ankle during physical activity, for the purposes of prevention of injury, protection of previously injured ligaments, or compensation for chronic instability.
An exceedingly wide variety of ankle orthosis designs have been developed, which have different closure mechanisms, strap configurations, and material characteristics. Prior art has recognized that restraint of abnormal ankle displacement requires orthosis elements that are firmly secured to both the leg and the foot, but the none of the linkage mechanisms between the leg and the foot segments described in the prior art effectively restrain excessive subtalar joint motion or combined rotary and translational displacement of talus, while simultaneously permitting unrestrained upward and downward movement of the foot in the sagittal plane. A review of the biomechanical function of the leg, ankle, and foot is essential for understanding of the mechanism by which a new and original pivoting strap system design provides an optimal level of ankle stability when incorporated into the structure of an ankle orthosis or shoe.
Movements of body segments have traditionally been defined by assuming that the proximal segment (closest to the body torso) remains stationary, while the non-weightbearing distal segment (furthest from the body torso) changes its position in space. The movements of the distal segment are defined in terms of cardinal planes, which correspond to the three dimensions of space and are perpendicular to one another. Isolated upward and downward movements of the foot in the sagittal plane (respectively termed dorsiflexion and plantar flexion) are associated with rotation around a horizontal axis that is perpendicular to the long axis of the foot. Isolated inward and outward tilting movements of the sole of the foot in the frontal plane (respectively termed supination and pronation) are associated with rotation around a horizontal axis that is aligned with the long axis of the foot. Isolated inward and outward displacements of the long axis of the foot in the transverse plane (respectively termed adduction and abduction) are associated with rotation around a vertical axis that is aligned with the long axis of the stationary leg.
Although the ankle is widely viewed as a single joint between the leg and foot, ankle motion actually involves extremely complex interrelationships between the articular surfaces and ligaments of the talocrural joint, subtalar joint, transverse tarsal joint, tarsometatarsal joints, and metatarsophalangeal joints. The motions of an individual joint, which are almost never confined to a single cardinal plane, are primarily determined by the geometric configuration of its articular surfaces. A joint's “functional” axis of rotation is an imaginary line in space, around which angular motion occurs. Thus, the “functional” axis of angular motion for a given joint has a spatial orientation that typically deviates to some degree from vertical and horizontal reference planes.
The talocrural joint (TCJ), which is comprised of the tibia, fibula, and talus, is widely referred to as the ankle joint. Most of the upward and downward movement of the foot results from TCJ motion, but most of the side-to-side movement of the foot results from motion between the talus and calcaneus in the subtalar joint (STJ). Although the TCJ and STJ function in a highly integrated manner, they represent two distinctly separate joints. Thus, the TCJ should be viewed as the “upper” ankle joint and the STJ as the “lower” ankle joint. The orientation of the functional axis of the TCJ closely corresponds to the lower tips of the bony protuberances on either side of the ankle (the tibial malleolus and the fibular malleolus). Although TCJ motion primarily occurs within the sagittal plane, and is referred to as dorsiflexion and plantar flexion, the TCJ functional axis has a somewhat oblique orientation that combines some degree of supination and adduction with plantar flexion, and some degree of pronation and abduction with dorsiflexion (FIGS. 1-4).
The inward rotation of the sole of the foot that results from STJ motion is referred to as inversion, whereas the reciprocal outward rotation is referred to as eversion. From an outer side view, the average inclination of the STJ functional axis relative to the sole of the foot approximates a 45-degree orientation (FIG. 5). The function of the STJ has been compared to that of a “mitered hinge” between the leg and the foot, which produces rotation of the segments on either side of the hinge in opposite directions (FIG. 6). During weightbearing, frictional force between the ground and the foot results in the transfer of torque to the leg. The “mitered hinge” effect of the STJ causes inversion to be coupled with external rotation of the leg, and causes eversion to be coupled with internal rotation of the leg (FIG. 7). Similarly, rotation of the leg upon a weightbearing foot results in the transfer of torque through the STJ to the forefoot.
Excessive inversion torque is the dominant injury producing force in 85% of all ankle sprains. The position of the foot at the moment of injury is typically a combination of plantar flexion and inversion. The anterior talofibular ligament (ATFL) is the weakest and most vulnerable component of the lateral ligament complex, and it is the first to be stressed by the typical ankle sprain mechanism. Numerous experts in ankle biomechanics have emphasized the importance leg external rotation as key factor contributing to disruption of the ATFL. If the foot is firmly fixed to the ground, such that there is minimal movement at the ground-sole interface, torque is concentrated on the linkage between the foot and leg. As the leg rotates externally upon the talus, the ATFL is subjected to tensile stress. Tearing and/or chronic elongation of the ATFL results in abnormal rotary mobility within the TCJ (anterolateral rotary instability: anterior translation of the anterolateral portion of the talus from the tibio-fibular socket as the leg rotates externally; FIG. 8).
To effectively prevent lateral ankle sprain, or chronic rotary displacement, an ankle orthosis must restrain excessive and abnormal coupled motions within the joints of the forefoot (tarsometatarsal joints and transverse tarsal joint) and hindfoot (STJ and TCJ). To be practical for use among athletes, the device must not significantly restrict upward and downward foot movements necessary for running and jumping (dorsiflexion and plantar flexion). The degree of resistance provided to a given ankle motion by a orthosis is determined by the following factors: 1) the geometric configuration of the elements of the device relative to the spatial orientation of the functional axis of motion, 2) the degree of stiffness or elasticity of the materials that comprise the orthosis elements, and 3) the degree of fixation of the device to both the leg and the foot segments.
When subjected to excessive inversion stress, the articular surfaces of the joints of the foot and ankle are distracted laterally and compressed medially. Several different strategies may be employed to resist such joint displacement. A “stirrup brace” incorporates medial and lateral components that are constructed from a relatively rigid plastic. Both components contribute to ankle stability, but have different biomechanical effects. The component that spans the medial joint surfaces acts like a “spacer bar” to resist medial compression. Assuming that the lateral component of a stirrup brace exerts pressure against the lateral surface of the ankle, it acts as a “buttress” to resist lateral distraction. Cloth adhesive tape applied to the ankle provides support through a different mechanism. When skillfully applied, strips of adhesive tape develop tension in response to distraction of the joints that they span, thereby acting like a “tether” that resists separation of its attachment points. Thus, tape may function like an “external ligament” that limits the displacement of underlying joint surfaces. Lace-up ankle braces, which are constructed from pliable fabric (e.g., nylon or vinyl) and are secured to the leg and foot segments by means of a system of eyelets and lacing (Hely, U.S. Pat. No. 5,067,486; Nelson, U.S. Pat. No. 4,237,874), do not conform to the ankle contours as closely as adhesive tape. The primary mode of protection for the lateral ankle ligaments provided by a lace-up ankle brace is probably derived from the manner in which the joints of the hindfoot are encased by material (i.e., a lateral buttress effect).
Ideally, an ankle support system should restrict excessive motion within the STJ, without significant restriction of motion within the TCJ. Because a wide range of upward and downward foot motion is clearly desirable for activities that involve running and jumping, some semi-rigid ankle brace designs have incorporated hinges on the medial and/or lateral aspects of the brace that are intentionally aligned with the approximate location of the TCJ axis (Bowman, U.S. Pat. No. 6,689,081; Miklaus et al., U.S. Pat. No. 5,209,722; Peters, U.S. Pat. No. 5,031,607; Peters, U.S. Pat. No. 6,053,884; Quinn et al., U.S. Pat. No. 5,971,946; Richie Jr., U.S. Pat. No. 6,602,215; Westin et al., U.S. Pat. No. 4,646,726). Fixation of brace components to the leg and foot segments is often provided by straps that incorporate Velcro hook and loop closure material. Many ankle braces incorporating adjustable-tension straps that link foot and leg components include a semi-rigid cuff and strap system that encircles the leg (Broadhurst, U.S. Pat. No. 4,982,733; Gilmour, U.S. Pat. No. 5,899,872; Hayashi, U.S. Pat. No. 6,056,713; Kenosh, U.S. Pat. No. 5,810,754; Peters, U.S. Pat. No. 6,053,884; Sutherland, U.S. Pat. No. 4,753,229; Westin et al., U.S. Pat. No. 4,646,726). To reduce interference with normal upward and downward movement of the foot, adjustable-tension straps are sometimes anchored to foot and/or leg brace components by means of a pivoting connection (Avon, U.S. Pat. No. 6,793,640; Baron, Des. 338,066; Bowman, U.S. Pat. No. 6,689,081; Montag, U.S. Pat. No. 5,472,411; Richie Jr., U.S. Pat. No. 6,602,215; Sutherland, U.S. Pat. No. 4,753,229; Westin et al., U.S. Pat. No. 4,646,726). An ankle support device is typically separate from the structure of the shoe within which it is worn, but support ankle systems may be embodied in either the form of an orthosis or incorporated into the structure of a shoe (Burns, U.S. Pat. No. 4,441,265; Kenosh, U.S. Pat. No. 5,810,754; Sutherland, U.S. Pat. No. 4,753,229; Townsend et al., U.S. Pat. No. 6,228,043).
Because motion of the talus is influenced by torque that is transferred from joints in the forefoot, restriction of excessive forefoot inversion is essential for optimal maintenance ankle stability. To control forefoot inversion, the support system must span the set of articulations between the talus, calcaneus, navicular, cuboid, and fifth ray, and it must be firmly anchored to both the leg and forefoot. The designs of most ankle support systems reflect a focus on enhancement of the stability of the hindfoot, without any attempt to control motion within the joints of the forefoot. However, there are a few notable exceptions. Westin et al. (U.S. Pat. No. 4,646,726) disclosed a design that incorporates an adjustable-tension oblique strap, which extends from the forefoot portion of a footplate component to a common junction with another vertical strap that is anchored to a leg cuff component. Kenosh (U.S. Pat. No. 5,810,754) disclosed a non-adjustable design that incorporates a “talofibular support portion” that is continuous in structure with the rest of the orthotic, which is clearly intended to provide a forefoot stabilization effect. Avon (U.S. Pat. No. 6,793,640) disclosed a device that is intended for control of ankle instability associated with paralysis, which incorporates a pair of obliquely-oriented adjustable straps on its medial and lateral aspects that span the joints of the forefoot and midfoot.